Carbon dioxide refrigeration system and a method of operating the refrigeration system

ABSTRACT

A refrigeration system including one or more first compressor(s) for compressing a carbon dioxide (CO2) refrigerant, a main heat rejection system for cooling the CO2 refrigerant, one or more high pressure expansion device(s) for reducing the pressure of the CO2 refrigerant, a receiver for storing the CO2 refrigerant, one or more high pressure expansion device(s), an evaporator and a receiver pressure regulating device. The refrigeration system further includes an auxiliary refrigeration system including an auxiliary compressor arranged to compress at least part of the CO2 refrigerant and thereafter to direct the compressed CO2 refrigerant to a heat rejection system.

The invention relates to a transcritical refrigeration system using carbon dioxide as refrigerant and having a main and an auxiliary refrigeration system and to a method of operating said refrigeration system.

BACKGROUND

Conventional refrigeration systems comprise a cycle having an evaporator, a compressor, a condenser and an expansion valve. The refrigerant is compressed by the compressor to a high pressure, high temperature mode, after which a phase-change is introduced in the condenser, in order to reject heat from the refrigerant. Thereafter, an expansion valve reduces the pressure and thereby reduces the temperature of the refrigerant. The low temperature refrigerant is then led through the evaporator in which it is utilized to cool the surroundings of the refrigerant, such as an interior space of the refrigerator. The refrigerant is thereafter after led to the compressor, and the cycle repeats. The refrigeration cycle contains a refrigerant, which can be a substance or a mixture of substances and may be a natural or synthetic refrigerant, such as Ammonia (NH3) also termed R-717, Chloro Fluoro Carbon (CFC) such as freon-12 also referred to as R-12, Hydro Carbon (HC) such as propane (R-290), Hydro Fluoro Carbon (HFC), Hydro Fluoro Olefin (HFO) or Carbon dioxide (CO2) also referred to as R-744.

It is an object of the present invention to improve the resilience of refrigeration systems, employing CO2 as refrigerant, in the event of shut down of the refrigeration cycle of the refrigeration systems.

BRIEF DESCRIPTION OF THE INVENTION

Carbon dioxide (CO2) is a naturally occurring substance, making up about 0.04% of the Earth's atmosphere and the use of CO2 refrigerant (also called R-744) is advantageous in that it does not contribute to ozone depletion. Additionally, release into the atmosphere from a CO2 refrigeration system has a negligible effect compared to other CO2 sources that are driving the global warming debate. CO2 refrigerant is not only climate friendly, but also it also generally non-toxic and has no flammability risk. It has also shown to allow for significant energy savings over other types of refrigerants.

A phase diagram of a refrigerant illustrates the pressures and temperatures at which the substance exist in either a solid, liquid, vapour or supercritical phase. In the supercritical phase, the liquid and the vapour phase of the substance is non-distinguishable and co-exist. The refrigerant is present as a supercritical fluid and cannot condense, as there is no phase change when heat is removed from a supercritical fluid while it is above the critical pressure and temperature. For CO₂, the critical point is a 73.8 bar and 31.1 degrees Celsius. CO₂ has a low critical temperature compared to other commonly used refrigerants; however, it requires higher operating pressures than other refrigerants.

Conventional refrigeration systems are mainly operated with a refrigerant in the subcritical region, i.e. below the critical point separating the super-critical and subcritical regions in the phase diagram.

The first aspect of the present invention, relates to a refrigeration system for transferring heat, wherein the refrigeration system comprises a main refrigeration system comprising

-   -   one or more first compressor(s) for compressing a carbon dioxide         (CO₂) refrigerant,     -   a main heat rejection system for cooling the CO₂ refrigerant         discharged from said first compressor(s),     -   one or more high pressure expansion device for reducing the         pressure of the CO₂ refrigerant discharged from the main heat         rejection system,     -   a receiver for receiving CO₂ refrigerant from said high pressure         expansion device,     -   one or more low pressure expansion device(s) for reducing the         pressure of liquid phase CO₂ refrigerant from the receiver,     -   an evaporator for evaporating the liquid phase part of the CO₂         refrigerant from the low pressure expansion device and         thereafter directing the evaporated CO₂ refrigerant to a suction         side of the first compressor(s),     -   a receiver pressure regulating device for regulating the         pressure of a vapour phase part of the CO₂ refrigerant from the         receiver and thereby directing vapour CO₂ refrigerant to the         suction side of the first compressor(s), the refrigeration         system further comprises an auxiliary refrigeration system         comprising     -   an auxiliary compressor arranged to compress at least part of         the vapour phase of the CO₂ refrigerant from the receiver and to         direct the compressed CO₂ refrigerant to a heat rejection         system, wherein the nominal maximal volume rate of flow of         compressed refrigerant of the auxiliary compressor is less than         10% of the nominal maximal volume rate of flow of compressed         refrigerant of the main compressor(s), such as less than 5%,         preferably less than 1% thereof, such as less than 0.5% thereof.

In an alternative definition of the present invention, the auxiliary compressor is instead of being defined in terms of its nominal maximal volume rate of flow of compressed refrigerant relatively to the nominal maximal volume rate of flow of compressed refrigerant of the main compressor(s), defined by the nameplate electrical power motor driving the auxiliary compressor being less than 10% of the electrical power of the motor(s) driving the main compressor(s), such as less than 5%, preferably less than 1% thereof, such as less than 0.5% thereof.

The refrigeration system may further be equipped with a receiver pressure-regulating device for regulating the pressure of a vapour phase part of the CO2 refrigerant from the receiver and thereby directing vapour CO2 refrigerant to the suction side of the first compressor(s). In case such receiver pressure-regulating device is present, the auxiliary compressor may be arranged to draw CO₂ refrigerant in the vapour phase from the receiver via the suction side of the first compressor(s), provided that the receiver pressure-regulating device is in an open state allowing the vapour to flow to the suction side of the first compressor(s).

Advantageously, by the present invention, the refrigeration system becomes more resilient than other known refrigeration systems, in that the system has incorporated an auxiliary refrigeration system, which is arranged to remove a surplus of vapourized CO₂ from the receiver during a non-operational period of the main refrigeration system, e.g. during a power failure thereof or when no cooling is required from the refrigeration system and an excessive pressure may build up in the receiver. Furthermore, by operating the auxiliary compressor during a non-operational period, much less energy is required as compared to operating the main compressor(s) for maintaining a suitable pressure in the receiver. In one or more embodiments, the auxiliary refrigeration system may utilize some components of the main refrigeration system in the auxiliary cycle.

A specific association between the volume of the system, the pressures and the temperatures of the refrigerant has to be controlled throughout the refrigeration cycle in order to obtain optimal refrigeration conditions. CO₂ has a coefficient of expansion, which is significantly higher than other commonly used refrigerants, and a small temperature increase of the refrigerant can therefore lead to a high-pressure increase. Therefore, especially the components of the refrigeration system, i.e. the pipe work, devices, equipment and tools, must be suitable for use with a broad range pressure in a CO₂ refrigeration system.

In order to reduce an occurrence of vapour entering the low-pressure expansion device, so called flash gas, a receiver is preferably introduced in the cycle prior to the low-pressure expansion device. The receiver may be arranged to separate the liquid and vapour parts of the refrigerant, so that the vapour part may be led to the first compressor, bypassing the evaporator and the low-pressure expansion device.

Additionally or alternatively, the refrigeration system may experience a local pressure increase, especially during off-periods. Therefore, the refrigeration system may be equipped with one or more pressure relief valves, for discharging CO₂ to the atmosphere. In one or more embodiments, the refrigeration system comprises one or more pressure relief valves arranged at location(s) along the main refrigeration cycle, e.g. along a line extending form the receiver may typically be equipped with such a pressure relief valve.

Additionally or alternatively, the refrigeration system, e.g. the receiver, may be fitted with insulation or with a separate cooling system, to ensure the temperature and pressure is maintained within a predetermined range during shut-down of the main refrigeration system.

However, alternatively or additionally, by the present invention, in the event that the main refrigeration system is turned off, the overflow of CO₂, caused by an increase in pressure and temperature, may be at least partly or fully utilized by the refrigeration system, by leading the refrigerant overflow through an auxiliary refrigeration system. In doing so parts of the main refrigeration system is bypassed, such as the first compressor, and the overflow of gaseous CO₂ is led directly to a heat rejection device, where heat can be rejected from the refrigerant. Advantageously, this allows for the overflow of CO₂ to be utilized by the system instead of going to waste through pressure relief valve, which would inevitably result in the need for the system to be refilled with CO₂.

In one or more embodiments, the refrigeration system may be arranged in a commercial cooling unit for cooling elements, such as air, within that unit, such as a refrigerator or freezer. The heat generated by the refrigeration system may also be utilized for heating.

In one or more embodiments, the refrigeration system may be arranged as a supercritical refrigeration system, in which the refrigerant in at least a part of the refrigeration system is in its supercritical phase.

In one or more embodiments, the one or more first compressor(s) are arranged to compress the refrigerant into a supercritical phase. In one or more embodiments, the heat rejection system(s) are arranged to reduce the temperature of refrigerant in a supercritical state.

In one or more embodiments, the one or more first compressor(s) are arranged to compress the carbon dioxide (CO₂) refrigerant from a vapour phase to a supercritical phase and wherein the main heat rejection system is arranged to cool supercritical CO₂ refrigerant discharged from said first compressor(s).

In one or more embodiments, the high-pressure expansion device(s) are arranged to reduce the pressure of the CO₂ refrigerant in the supercritical phase, after being discharged from the main heat rejection system, wherein the high-pressure expansion device is further arranged to decompress the CO₂ refrigerant to a subcritical state.

In one or more embodiments, the receiver is arranged to receive subcritical CO₂ refrigerant from said high-pressure expansion device(s). The refrigerant is preferably naturally separated into a vapour and a liquid part in the receiver.

In one or more embodiments, the auxiliary compressor is arranged to compress at least part of the vapour phase of the CO₂ refrigerant from the receiver or from the suction side of the first compressor(s) to a supercritical phase and thereafter to direct the compressed CO₂ refrigerant to a heat rejection system.

In an exemplary refrigeration system according to the invention, the main refrigeration system comprises

-   -   one or more first compressor(s) for compressing a carbon dioxide         (CO₂) refrigerant from a vapour phase to a supercritical phase,     -   a main heat rejection system for cooling the CO₂ refrigerant in         the supercritical phase discharged from said first         compressor(s),     -   one or more high pressure expansion device for reducing the         pressure of the CO₂ refrigerant in the supercritical phase,         after being discharged from the main heat rejection system,         wherein the high pressure expansion device is arranged to         decompress the CO₂ refrigerant to a subcritical state,     -   a receiver for receiving subcritical CO₂ refrigerant from said         high pressure expansion device,     -   one or more low-pressure expansion device(s) for reducing the         pressure of the liquid phase CO₂ refrigerant from the receiver,     -   an evaporator for evaporating the liquid phase part of the CO₂         refrigerant from the low pressure expansion device and         thereafter directing the evaporated CO₂ refrigerant to a suction         side of the first compressor(s),     -   a receiver pressure regulating device for regulating the         pressure of the vapour phase part of the CO₂ refrigerant from         the receiver and thereby directing vapour CO₂ refrigerant to the         suction side of one or more of the first compressor(s), the         refrigeration system further comprises an auxiliary         refrigeration system comprising     -   an auxiliary compressor arranged to compress at least part of         the vapour phase of the CO₂ refrigerant from the receiver or         from the suction side of the first compressor(s) to a         supercritical phase and thereafter to direct the compressed CO₂         refrigerant to a heat rejection system,         wherein the auxiliary refrigeration system is arranged to be         powered by an auxiliary power supply separate from the power         supply of the main refrigeration system.

In one or more embodiments, the refrigeration system may be a so called simple transcritical system, a single-stage system, a simple booster system, a cascade system or a system with a multi-injector solution or a combination of the above. Independent of the type of refrigeration system, the auxiliary refrigeration system is arranged to extract vapour CO₂ from e.g. one or more receiver(s) or the suction side of the first compressor, decompress it, and discharge it to a heat rejection system. The refrigeration system may comprise one or more auxiliary refrigeration system and/or one or more auxiliary compressors, e.g. arranged in parallel. Preferably, the refrigeration system is a refrigeration system, which is capable of using CO₂ in transcritical mode. In one or more embodiments, the cascade system may be a hybrid cascade, utilizing CO₂ as low stage refrigerant and a different refrigerant, e.g. HFC og HC as high stage refrigerant. It should be noted that refrigeration systems suitable for operating in transcritical conditions according to the present invention in general also might be operated in critical or subcritical conditions.

Advantageously, the auxiliary refrigeration system may be fitted into already existing refrigeration systems, especially as the auxiliary refrigeration cycle may utilise at least parts of the main refrigeration system, such as at least some of the piping of the a part of the main refrigeration system.

In one or more embodiments, the receiver is arranged as a storage device capable of storing the refrigerant and/or capable of dividing the refrigerant in to a vapour and liquid phase. The receiver may be arranged in fluid communication one or more lines on a low pressure side and or a high pressure side of the refrigeration system, such as through a line comprising the receiver pressure regulating device and extending between the suction side of the first compressor and the receiver, thereby bypassing the evaporator and the low pressure expansion device. Additionally the receiver is preferably directly connected by a line to the low-pressure expansion device. One or more lines for discharging CO₂ from the receiver may be controlled by one or more control systems so that the amount of refrigerant, and thereby the capacity of the refrigeration cycle can be varied.

In one or more embodiments, the receiver pressure-regulating device is preferably arranged to regulate the amount of vapour phase CO₂ present in the receiver. In the event, that the amount needs to be reduced, the pressure regulating device may be operated to allow for vapour phase CO₂ to be directed to the suction side of the first compressor, bypassing the evaporator and the low pressure expansion device. The receiver pressure-regulating device may comprise an electronic valve, such as a gas bypass valve, e.g. an electronically operated CCM or a high-pressure valve e.g. a CCMT regulating valve or a motor operated valve such as an ICMTS valve. Alternatively, the receiver pressure-regulating device may comprise a receiver pressure-regulating compressor, such as an IT compressor, arranged between the receiver and the heat rejection device. The receiver-regulating compressor is preferably configured to regulate the amount of vapour phase CO₂ in the receiver during normal operation of the refrigeration system, i.e. when the main refrigeration system is powered and activated. The receiver-regulating compressor is typically fed by the same power supply as the main refrigeration system. In one or more embodiments, the main refrigeration system may comprise both a receiver pressure-regulating valve and a receiver pressure-regulating compressor.

In one or more embodiments, the receiver is in fluid communication with both the auxiliary and the main refrigeration system, to each by at least one line. The receiver may enclose a cavity in the interior of the receiver for storing the refrigerant. The cavity may separate phases of the refrigerant, so that the liquid part of the refrigerant is stored near the bottom of the cavity while the vapour part of the refrigerant is stored near the top of the cavity. The top of the cavity may then at least be in fluid communication with the suction side of the first compressor and the bottom of the cavity may be in fluid communication with the low-pressure expansion device.

In one or more embodiments, the evaporator is arranged so as to introduce heat to be absorbed by the refrigerant, causing the temperature of the refrigerant to increase from an inlet to an outlet of the evaporator. In one or more embodiments, the evaporator is arranged to operate at a refrigerant temperature and pressure in the subcritical range, preferably the refrigerant is in liquid phase. The evaporator may then preferably or exclusively be fed with refrigerant in the liquid phase. In one or more embodiments, the refrigeration system comprises a plurality of evaporators, such as two or three evaporators, arranged between the receiver and the first compressor. The evaporators may be arranged in series and/or in parallel.

In one or more embodiments, the first compressor and/or the auxiliary compressor is arranged to increase the pressure and the temperature of the CO₂ refrigerant, such as to a pressure above the critical point of the refrigerant. The first compressor and/or the auxiliary compressor is preferably arranged to be fed with CO₂ refrigerant in the vapour phase. In one or more embodiments, the first compressor may be a MT-type compressor. In one or more embodiments, the refrigeration system comprises a plurality of first compressors and/or a plurality of auxiliary compressors, such as two or three first compressors and/or auxiliary compressors. The first compressors may be arranged in series and/or in parallel between the evaporator and the heat rejection device. In one or more embodiments, the auxiliary compressor(s) may be arranged between the receiver, the receiver pressure regulating device or the evaporator and a heat rejection system. The auxiliary compressors may be arranged in series or in parallel therein between.

In one or more embodiments, the auxiliary compressor is arranged to direct the compressed CO₂ refrigerant to the main heat rejection system. In alternative embodiments, the auxiliary refrigerating system comprises an auxiliary heat rejection system to which the compressed CO₂ refrigerant from auxiliary compressor is directed.

Advantageously, this allows for the compressed CO₂ refrigerant, such as compressed CO₂ refrigerant in the subcritical phase, to be cooled by a heat rejection system, so that the temperature of the refrigerant is decreased.

In one or more embodiments, the main heat rejection system and the auxiliary heat rejection system may preferably be powered by separate power supplies, so that the auxiliary heat rejection system may be powered by an auxiliary power supply in the case of a power failure of a main power supply to the main refrigeration system. The auxiliary heat rejection system may be arranged in series with the auxiliary compressor and parallel to the heat rejection system of main refrigeration system. In one or more embodiments, the refrigeration system may be arranged to allow the refrigerant on the pressure side of the auxiliary compressor to be directed either through either the main heat rejection system of the main refrigeration system or the auxiliary refrigeration system or both.

In one or more embodiments, the main heat rejection system and/or the auxiliary heat rejection system may be a gas cooler equipped with means for either air or water cooling and suitable for a CO₂ refrigeration system, such as a transcritical CO₂ refrigeration system. The heat rejection system(s) may be arranged to functions as a condenser in subcritical operation and as a gas cooler in transcritical operation. In the transcritical operation mode the heat rejection system(s) may also be referred to as gas cooling system(s). The heat rejection system(s) may be arranged to operate at a refrigerant temperature and pressure in excess of the refrigerant's critical point temperature and pressure and to decrease the temperature of the refrigerant, when the refrigerant is in a trans-critical mode, in which pressure and temperature are no longer dependent on each other. In one or more embodiments, the temperature continuously decreases as the refrigerant travels through the heat rejection system(s).

In one or more embodiments, the main heat rejection system is arranged to allow for cooling of the CO₂ refrigerant discharged from the auxiliary compressor and to allow the CO₂ refrigerant to be led through the main heat rejection system, when the main heat rejection system is off.

Advantageously, the auxiliary refrigeration system is thereby arranged to utilize the components of the main refrigeration system, thereby reducing the amount of components needed in the auxiliary refrigeration system. Furthermore, it may provide a more energy efficient system in which the cooling capacity of the main heat rejection system is utilized instead of going to waste.

In one or more embodiments, the main heat rejection system is preferably of a capacity large enough to be able to sufficiently cool the refrigerant discharged from the auxiliary compressor. The discharge is typically low compared to the discharge received during normal operation, from the first compressor, and therefore the low amount of discharge can be cooled sufficiently, even when the main heat rejection system is not powered.

In one or more embodiments, the refrigeration system comprises a plurality of high-pressure expansion devices, such as two or three high-pressure expansion devices, e.g. arranged in series with a heat rejection system of the refrigeration system. In one or more embodiments, the refrigeration system comprises a plurality of low-pressure expansion devices, such as two or three low-pressure expansion devices, e.g. arranged in series with the evaporator.

In one or more embodiments, the auxiliary refrigeration system is arranged to operate and utilize the high and/or low expansion device(s) of the main refrigeration system.

In one or more embodiments, the auxiliary refrigeration system further comprises one or more auxiliary high-pressure expansion device for expanding the CO₂ refrigerant received from the heat rejection system.

Advantageously, by having an auxiliary high-pressure expansion device, the auxiliary refrigeration system may become increasingly operationally independent of the main refrigeration system. This may also provide a larger degree of control of the refrigeration cycle of the operation of the auxiliary refrigeration system. In one or more embodiments, the refrigerant may be received from the main heat rejection system, from the auxiliary heat rejection system or both.

In one or more embodiments, the auxiliary high-pressure device may be arranged between the receiver and the heat rejection system, such as in parallel with the one or more high-pressure expansion device of the main refrigeration system.

In one or more embodiments, the auxiliary high-pressure expansion device is powered independently from the high-pressure expansion device of the main refrigeration system, e.g. the auxiliary high-pressure expansion device may be powered by an auxiliary power supply.

In one or more embodiments, the auxiliary high pressure expansion device and/or the high pressure expansion device and/or the low pressure expansions device may comprise an electronic expansion valve, such as a AKVH valve or CCMT valve or an ICMTS valve or a CCM valve or a thermostatic valve or a fixed orifice device, such as a capillary tube. In one or more embodiments, the auxiliary high-pressure expansion device, the high-pressure expansion and the high-pressure device is a CCMT or ICMTS valve.

In one or more embodiments, the high-pressure expansion device and the auxiliary high-pressure expansion device are arranged to reduce the refrigerant pressure to about 60 bar, such as between 55 and 65 bar. The refrigerant pressure prior to the high-pressure expansion device(s) may be in the supercritical region, such as between 80 and 100 bar, preferably between 85 and 95 bar.

In one or more embodiments, the CO₂ refrigerant pressure is equal to or higher than the critical pressure of the CO₂ refrigerant before being decompressed by the high-pressure expansion device or the auxiliary high-pressure expansion device.

In one or more embodiments, the receiver, the evaporator, the first compressor, the main heat rejection system and the expansion devices are connected in series in a main refrigeration cycle in which the CO₂ refrigerant is circulating. In one or more embodiments, the refrigeration system is refrigeration cycle having a low-pressure side, concentrated between the outlet of the low-pressure expansion device and the inlet of the first compressor and a high-pressure side between the outlet of the first compressor and the inlet of the low-pressure expansion device. The refrigeration system preferably further comprises one or more intermediate lines extending from the receiver to the first compressor, bypassing the low-pressure expansion device and the evaporator. The intermediate region may be a part of the low-pressure side, such that the high-pressure side extends from the first compressor to the inlet of the high-pressure expansion valve. Alternatively, the refrigeration system may comprises a low pressure side between the low pressure expansion device, the receiver pressure regulating device and the first compressor, an intermediate low pressure side extending between the high pressure expansion device to the receiver pressure regulating valve and the low pressure expansion device, and a high pressure side between the first compressor and the high pressure expansion device. In one or more embodiments, along the cycle encompassed by the low-pressure side and if present, the intermediate low pressure, side the refrigerant is preferably in its sub-critical range, while it is preferably in its transcritical range along the cycle encompassed by the high-pressure side.

In one or more embodiments, the low-pressure side, e.g. the receiver may be in direct fluid communication with the high-pressure side of the refrigeration cycle, via the auxiliary compressor. In one or more embodiments, the auxiliary compressor may be arranged in fluid communication with and in between the low-pressure side, or the intermediate low-pressure side, and the high-pressure side of the refrigeration system.

In one or more embodiments, the high pressure side may comprise a first high pressure side between the first or auxiliary compressor and the inlet of a heat rejection system in which the CO₂ refrigerant is of higher pressure than between a second high pressure side extending between the heat rejection system and the high pressure expansion device(s).

In one or more embodiments, the auxiliary refrigeration system may be arranged with one or more lines extending from a location on the main refrigeration cycle to the auxiliary compressor, the point on the main refrigeration cycle preferably being a location in which vapour CO₂ may form, especially when the main refrigeration system is turned off, e.g. at locations after the evaporator or the receiver, such as at the suction side of the main compressor.

In one or more embodiments, the auxiliary refrigeration system comprises a line enabling refrigerant flow from the top of the receiver cavity through the auxiliary compressor and back to the main refrigeration system at a point between the first compressor and the main heat rejection system, such that vapour phase refrigerant can be lead directly towards the main heat rejection system of the main refrigeration system. Alternatively, the vapour phase refrigerant may be lead through an auxiliary heat rejection system, which is not part of the cycle of the main refrigeration system but may be a part of the auxiliary refrigeration system.

In one or more embodiments, when the main refrigeration system is activated, the typical operating pressure is between 35-45 bar in the receiver, between the high-pressure expansion device and the low-pressure expansion device/receiver pressure regulating device. In one or more embodiments, when the main refrigeration system is activated the typical operating pressure is between 10-35 bar at the evaporator, between the low-pressure expansion device and the compressor. In one or more embodiments, the intermediate low-pressure side extending between the receiver and the compressor has a typical operating pressure of 35-65 bar. In one or more embodiments, when the main refrigeration system is activated, the operating pressure is above 70 bar, such as at least 73 bar, or between 73-100 bar, such as between 85-95 bar at the heat rejection system, e.g. between the compressor and to the high pressure expansion device when the refrigerant is in a transcritical mode. In one or more embodiments, the cavity of the receiver have a pressure during operation of the main refrigeration system which is less than the high pressure side and higher than the low pressure side of the refrigeration system. In one or more embodiments, when the main refrigeration system is activated, the operating pressure is equal or below 70 bar at the heat rejection system,

In one or more embodiments, when the auxiliary refrigeration system is activated, the receiver cavity is at an operating pressure above 35 bar, such as between 35 and 60 bar. The auxiliary refrigeration system may be arranged to maintain a pressure between 30 and 45, or preferably between 35 and 40 bar in the receiver. Advantageously, the system is thereby prevented from reaching operating pressures below 35 bar, which could cause the formation of frost.

In one or more embodiments, the first compressor and/or the auxiliary compressor may comprise one or more single-stage or multi-stage compressors. The first compressor and/or the auxiliary compressor may be a rotary compressor, a reciprocating compressor, a scroll compressor, a screw compressor or a centrifugal compressor or a combination thereof, e.g. arranged in parallel or tandem configuration.

In one or more embodiment, the auxiliary compressor of the auxiliary refrigeration system may have a smaller compressing power than the first compressor, such as about 10-1500 times smaller compressing power, or such as between 100-1000 times smaller than the first compressor. In one or more embodiments, the auxiliary compressor may have a capacity of between 0.5-2 kW such as between 0.5-1 kW. In one or more embodiments, the capacity of the first compressor may be between 50-700 KW, such as between 100-500 kW.

The overflow of CO2, which is to be handled by the auxiliary compressor, is typically of small amounts, meaning that the auxiliary system can advantageously be sufficient even with a small compressor capacity.

In one or more embodiments, the auxiliary compressor may be arranged to continuously maintain stable operation of the refrigeration cycle of the refrigeration system, e.g. during maintenance of e.g. the first compressor(s).

In one or more embodiments, the refrigeration system is arranged so that the auxiliary refrigeration system functions as an emergency back-up system, powered by an auxiliary power supply being separate from a main power supply powering the main refrigeration system. This allows for the emergency back-up system to kick in and be activated, even in the case of a failure or a cut-off of power to the main refrigeration system. In one or more embodiments, the auxiliary power supply may also be arranged to power parts of the main refrigeration system, such as the main heat rejection system and/or the high-pressure expansion valve of the main refrigeration system. The auxiliary refrigeration system may be powered by a generator and/or an uninterruptible power supply (UPS), e.g. an energy storage device, such as one or more electrical batteries. In one or more embodiments, the auxiliary and the main refrigeration systems are both powered by a common electrical grid, and the auxiliary refrigeration system is arranged to provide a refrigeration cycle when the main refrigeration system is turned off e.g. during maintenance of the main refrigeration system, but not during power failure on the common electrical grid.

In one or more embodiments, the overflow of CO₂ may be characterised as being present when the refrigerant pressure reaches a pressure above a predetermined maximum pressure value for the system, which may be determined based on the receiver and system capability. It is the purpose of the auxiliary refrigeration system to keep the pressure below the predetermined maximum pressure during periods in which the main refrigeration system is not in operation.

In one or more embodiments, the refrigeration system comprises one or more control system for controlling the operation of the auxiliary refrigeration system, such as the auxiliary compressor, based on one or more inputs, such as one or more inputs representing the pressure and/or temperature of the refrigeration system.

In one or more embodiments, the refrigeration system comprises one or more control systems for controlling the operation of the refrigeration system. In one or more embodiments, the control system comprises means for controlling the operation of the auxiliary refrigeration system, either by manual and/or semi- or fully automatic implemented control settings. In one or more embodiments, a control system of the refrigeration system may be arranged to automatically activate, i.e. turn on, the auxiliary refrigeration system, upon registration of a value, which is determined, e.g. by the control system, to be above a predetermined threshold value. This value may be representative of the pressure and/or temperature present in the refrigeration system, such as on the suction side of the compressor, such as in the interior of the receiver. The interior of the receiver may be defined as the cavity enclosed by the receiver, in which the refrigerant is stored.

In one or more embodiments, the refrigeration system comprises one or more measuring devices for measuring a value representative of the pressure and/or temperature of the refrigeration system and providing an input accordingly to one or more control systems of the refrigeration system.

In one or more embodiments, the control system may be arranged to control the refrigeration system based on one or more inputs from one or more measuring devices. The inputs may comprise information relating to a value detected by the measuring device, such as a value representing the pressure and/or temperature on the suction side of the compressor(s), e.g. the receiver pressure and/or temperature. The measuring device(s) may be at least arranged to measure properties of the refrigerant present in the pipelines connected at the suction side of the compressor(s) such at the suction side of the auxiliary compressor, which properties can represent the pressure and/or temperature of the refrigerant.

Similarly, in one or more embodiments, the control system may be arranged to control the receiver pressure-regulating device based on a detected value representing the pressure and/or temperature of the refrigeration system, such as in the receiver. The control system may be arranged to at least partially open the receiver pressure-regulating device in order to release pressure, by allowing refrigerant in vapour form to be led through the receiver pressure-regulating device to the suction side of the compressor(s). In one or more embodiments, as the main refrigeration system is powered off the receiver pressure regulating device may be arranged to automatically switch to an open position allow refrigerant flow.

In one or more embodiments, the refrigeration system is fitted with one or more pressure measuring device for detecting a value representative of the pressure within the refrigeration system, such as within the receiver and/or within the lines connected to a compressor, on the suction side, and providing the detected value to the control system. The pressure-measuring device may be arranged to continuously detect a value representing the pressure and continuously providing the detected value to the control system.

In one or more embodiments, the control system may be arranged to activate the auxiliary refrigeration system until a pressure below the predetermined threshold value is detected in the receiver.

In one or more embodiments, the predetermined threshold value may be between 35 and 50 bar.

In one or more embodiments, the auxiliary refrigeration system comprises a control means, such as one or more valves, for controlling a flow of CO₂ refrigerant from the receiver to the auxiliary compressor.

In one or more embodiments, the auxiliary refrigeration system comprises a control means, such as one or more valves, for controlling a flow of CO₂ refrigerant from the suction side of the first compressor to the auxiliary compressor. In one or more embodiments, the suction side is arranged on the low-pressure side of the compressor, e.g. between the evaporator and the compressor. In one or more embodiments, the control means are arranged to automatically transfer into an open state upon registration of a refrigerant pressure value above a pre-determined threshold pressure value. The control means may additionally be arranged to automatically transfer into a closed state upon registration of a refrigerant pressure value below a pre-determined threshold value.

In one or more embodiments, the auxiliary refrigeration system comprises control means having one or more pulse width modulation (PWM) valves, and wherein a control system of the refrigeration system is arranged to control the operation of the PWM valve based on one or more inputs representing the pressure and/or temperature of refrigeration system.

In one or more embodiments, the refrigeration system is fitted with a pulse width modulation valve on the suction side of the auxiliary compressor, such as between the receiver and the compressor, between the receiver regulating device and the compressor and/or between the evaporator and the auxiliary compressor. A control system of the refrigeration system may be arranged to control the pulse width modulation (PWM) valve based on pre-set control settings and/or based one or more detected values from the measuring device, e.g. such as the most recently detected value from the measuring device. In one or more embodiments, the PWM valve may be set to transfer between and open and close state at a pre-set frequency. The control system may be arranged to control the total time period for the activation of said PWM valve.

In one or more embodiments, the refrigeration system comprises a pressure switch arranged to be allow refrigerant flow to the auxiliary compressor when the pressure within the refrigeration system is within a first predetermined range of pressures and prevent refrigerant flow to the auxiliary compressor when the pressure is within a second predetermined range of pressures. The pressure may be the pressure of the receiver or the suction side of the refrigeration system. The first and second predetermined ranges may be modifiable by the user e.g. via a control system of the refrigeration system arranged to control the pressure switch. In one or more embodiments, the pressure switch may be arranged to be operable into and open state and closed state, or an intermediate state, by an operator of the refrigeration system.

In one or more embodiment, the refrigeration system comprises one or more oil separator device(s) for separating lubricant oil from the refrigerant. The refrigeration system may comprise an oil separator arranged to separate the oil from the refrigerant and send it to the first compressor(s) and/or the auxiliary compressor. The oil separator may be arranged connected to the compressors by independent and separate valves, such that the auxiliary compressor can suck in oil even when the main refrigeration system is not powered. The refrigeration system may additionally or alternatively comprise an auxiliary oil separator arranged to separate lubricant oil from the refrigerant and send it to the auxiliary compressor. In one or more embodiments, the compressors of the refrigeration system are arranged to be used with mineral oil as lubricant.

Advantageously, this allows for the compressor(s) to continuously be fed with lubricant, whereby wear and tear on the compressor parts are reduced and the lifetime of the compressor(s) is increased. Additionally, the oil separators reduce the risk of oil film formation on other parts of the refrigeration system, such as in the evaporator and heat rejection device, which can lead to decrease in heat transfer. It may also alter correct reading by any sensors within the system and incorrect operation of valves etc.

According to preferred embodiments, the auxiliary refrigeration system is arranged to be powered by an auxiliary power supply separate from the power supply of the main refrigeration system. Thus, the auxiliary refrigeration system may be powered fully independently from the main refrigeration system, such that in the event of power failure, the refrigeration system is still operable.

The second aspect of the present invention relates to a method of operating a refrigeration system in the event of the main refrigeration system of the refrigeration system being off, wherein the refrigeration system is according to one or more embodiments of the first aspect of the invention and wherein the method comprises the steps of:

-   -   detecting a value representing pressure and/or temperature of         the refrigeration system, such as the receiver pressure, by a         measuring device and providing an input accordingly to a control         system,     -   controlling the operation of the auxiliary compressor and if         present, the control means of the auxiliary refrigeration         system, by the control system based on said input.

Advantageously, the refrigeration system may thereby be maintained at a safe operational level, so that release of refrigerant to the atmosphere can be significantly reduced or prevented, and so that the components and elements, such as the piping, of the refrigeration system, are not damaged. Additionally, the pressure may be reduced by utilizing the overflow of CO₂ in the auxiliary refrigeration system.

In one or more embodiments, the method comprises the step of allowing a transfer of CO₂ refrigerant to the suction side of auxiliary compressor at values above a pre-set value. The value may be a refrigerant pressure value, such as the refrigerant pressure value present in the receiver. The method may further comprise the step of automatically activating the auxiliary compressor in order to compress the CO₂ refrigerant and directing the compressed CO₂ refrigerant to a heat rejection system. The automatic activation of the auxiliary compressor may be based on an input representing an open state of a valve or switch arranged to restrict or allow passage of CO₂ refrigerant from the receiver or the suction side of the first compressor to the auxiliary compressor.

In one or more embodiments, the value may be detected by a measuring device arranged to measure pressure and/or temperature of the interior of the refrigeration system, e.g. receiver or suction side of the auxiliary and/or the first compressor, and provide an input accordingly, to a control system which may be arranged to automatically control the operation of the auxiliary compressor and the control means based on said input, such that if the input is above a pre-determined threshold value, the control system will open the control means and activate the auxiliary compressor.

In one or more embodiments, the open state of the control means and the activation time of the auxiliary compressor may be automatically and continuously controlled based on most recently received input(s). The control system may be arranged to transfer the control means into a closed state and de-activate the auxiliary compressor once the detected value is below the predetermined threshold value.

In one or more embodiments, a control system of the refrigeration system may be arranged to control an auxiliary heat rejection system and/or an auxiliary high pressure expansion device of the refrigeration system, such as to activate and de-activate the auxiliary heat rejection system and/or the high pressure expansion device in accordance with the activation and de-activation of the auxiliary compressor.

In one or more embodiments, the control system is arranged to turn of the auxiliary refrigeration system once the main refrigeration system is turned back on.

DRAWING

Aspects of the present disclosure will be described in the following with reference to the figures in which:

FIG. 1 shows a schematic diagram of a refrigeration system according to an embodiment of the invention,

FIG. 2 shows a schematic diagram of a refrigeration system according to an embodiment of the invention,

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawing. In a first and second embodiment, a refrigeration system 1 using CO₂ as refrigerant 2 is shown in FIG. 1 and FIG. 2 . The refrigeration system 1 comprises a first compressor 3, a main heat rejection system 4, a low-pressure expansion device 7, a receiver 6, a high-pressure expansion device 8 and an evaporator 5 sequentially connected to each other. The refrigeration system 1 is arranged as a refrigeration cycle 20, which when viewed along a counter-clockwise direction in FIGS. 1-2 , allows for the refrigerant 2 to travel along a first line 21 from the receiver 6 to the low pressure expansion device 7, along a second line 22 from the low pressure expansion device 7 to the evaporator 5, along a third line 23 from the evaporator 5 to the first compressor 3, along a fourth line 24 from the first compressor 3 to the main heat rejection system 4, along a fifth line 25 from the main heat rejection system 4 to the high pressure expansion device 8 and along a sixth line 26 from the high pressure expansion device 8 to the receiver 6. This does not exclude, that there may be additional elements, such as devices and/or components, which may be arranged between the shown elements of the refrigeration system 1 of FIGS. 1-2 , e.g. valves, such as cut off valves and check valves may be arranged at one or more locations on the cycle 20.

Additionally, the refrigeration system 1 may further comprise a seventh line 27, an eight line 28 and a receiver pressure regulating device 12 for receiver pressure control, wherein the seventh line 27 is arranged from the receiver to the receiver pressure regulating device 12 and the eight line 28 is arranged form the receiver pressure regulating device 12 to the third line 23 or to the first compressor 3. Additionally, valves, such as GBC and/or NRV valves may also be fitted in the seventh and/or eight line.

FIG. 1 further shows the refrigeration system 1 comprising an auxiliary refrigeration system comprising an auxiliary compressor 10 arranged in fluid communication with the receiver 6 at the suction side and in fluid communication with the main heat rejection system 4 at the pressure side. The auxiliary refrigeration system may have a ninth line 29 extending from the seventh line 27 to an auxiliary compressor 10 and a tenth line 30 extending from the auxiliary compressor 10 to the fourth line 24. In one or more embodiments, the auxiliary compressor 10 is connected to the high pressure side of the main refrigeration cycle of the refrigeration system 1 such that at least the low pressure expansion device 7, the evaporator 5 and the first compressor 3 and in some embodiments, also the receiver pressure regulating device 12 is bypassed. Alternatively, the auxiliary compressor 10 may be connected directly between the suction side of the first compressor 4, such as by a line extending from the 8th (28) or the 3rd (23) line to the auxiliary compressor 10.

In one or more embodiments, and as shown in FIG. 1 , the refrigeration system 1 may additionally comprise an auxiliary high-pressure expansion device 9 connected in parallel with the high-pressure expansion device 8 of the main refrigeration system, which may be arranged for pressure control. The auxiliary high-pressure expansion device 9 may be connected to the fifth line 25 on the inlet side by an eleventh line 31 and to the sixth line 26 on the outlet side by a twelfth line 32.

The refrigeration system 1 may preferably be arranged so that the auxiliary refrigeration system is powered independently from the main refrigeration system, such that the auxiliary refrigeration system may turned on, either manually or automatically by a control system, even when there is no power being feed to the main refrigeration system. In the event of power failure or cut of power to the main refrigeration system, e.g. due to maintenance etc. it is likely that the refrigerant 2 present in the main refrigeration system, especially in the receiver 6, due to the relatively large volume, will experience an increase in temperature and thereby also in pressure. The pressure increase produced during the main refrigeration system off-period may be directed through the auxiliary compressor 10 to the main refrigeration cycle, i.e. through the main heat rejection system 4 and back towards the receiver 6. Even through the main heat rejection system 4 is not powered; it is preferably of a cooling capacity to allow sufficient cooling of the refrigerant 2. After the main heat rejection system 4, the refrigerant 2 is preferably led through the auxiliary high-pressure expansion device 9, where after the refrigerant 2 is returned and collected in the receiver 6. In one or more embodiments, parts of the main refrigeration system, such as the fourth 24, fifth 25 and/or the sixth lines 26 or a combination thereof may be passively arranged to lead the refrigerant 2 through the refrigeration system 1, even when the main refrigeration system is not powered.

FIG. 2 shows a second embodiment of the invention. The refrigeration system 1 of FIG. 2 is similar to the refrigeration system 1 of FIG. 1 but differs in that the auxiliary refrigeration system 10 further comprises an auxiliary heat rejection system 11. In this embodiment, the auxiliary system 10 also bypasses the main heat rejection system 4 of the main refrigeration system. As shown in FIG. 2 the auxiliary compressor is connected to the receiver 6 by a ninth line 29 extending from the seventh line 27 to the auxiliary compressor 10, so that the receiver pressure regulating device 12 is bypassed. Alternatively, in one or more embodiments, the auxiliary compressor 10 may be connected by a line extending from the auxiliary compressor 10 to the suction side of the first compressor 4, such as to the eight line 28 or the third line 23. The receiver pressure regulating device 12 may be arranged to allow flow of refrigerant 2 during operation of the auxiliary refrigeration system.

As shown in FIG. 2 , the auxiliary compressor 10 may be connected by a line 30 to the auxiliary heat rejection system 11 and by a thirteenth line to the inlet side of a high pressure expansion device 8, 9. The auxiliary refrigeration cycle is further enabled by the fifth line 25, sixth line 26, eleventh line 31, and/or twelfth line 32, in order to connect a high pressure expansion device 8, 9 and thereafter to the receiver 6.

In one or more embodiments, the auxiliary refrigeration system 10 may be connected to the main refrigeration system at the fifth line 25 as shown in FIG. 2 , or it may be bypassing the fifth line 25 and connected directly to a high pressure expansion device 8, 9.

In one or more embodiments, the auxiliary refrigeration system may utilize the sixth line 26 to connect to the receiver 6, as shown in FIGS. 1 and 2 , or it may alternatively be connected by a separate line e.g. a line from the auxiliary high pressure expansion device 9 to the receiver 6. In one or more embodiments, the auxiliary refrigeration system is only connected to the main refrigeration system at the receiver 6.

The receiver 6 is preferably arranged to provide for two phases of the refrigerant 2 to be present in the receiver 6 and for allowing the vapour part to be recompressed either through the main refrigeration system or through the auxiliary refrigeration system 10 e.g. when the main refrigeration system is not powered. Simultaneously, the liquid part may be retained in the receiver 6 when the main refrigeration system is not powered on or it may be led through the main refrigeration system, when the main refrigeration system is on.

In one or more embodiments, the refrigeration system 1 may further be equipped with an oil extraction device for the first compressor 3 and arranged between the first compressor 3 and the main heat rejection system 4, e.g. such as immediately before an inlet of the main heat rejection system 4. The refrigeration system 1 may additionally or alternatively be equipped with an auxiliary oil extraction device for the auxiliary compressor 10 arranged between the heat rejection device 4, 12 and the auxiliary compressor 10.

REFERENCE LIST

-   -   1 refrigeration system     -   2 refrigerant, e.g. CO₂ refrigerant     -   3 first compressor     -   4 main heat rejection system     -   5 evaporator     -   6 receiver     -   7 low pressure expansion device     -   8 high pressure expansion device     -   9 auxiliary high pressure expansion device     -   10 auxiliary compressor     -   11 auxiliary heat rejection system     -   12 receiver pressure regulating device     -   20 refrigeration cycle     -   21 first line     -   22 second line     -   23 third line     -   24 fourth line     -   25 fifth line     -   26 sixth line     -   27 seventh line     -   28 eight line     -   29 ninth line     -   30 tenth line     -   31 eleventh line     -   32 twelfth line     -   33 thirteenth line 

1. A refrigeration system for transferring heat, wherein the refrigeration system comprises: a main refrigeration system comprising: one or more first compressors for compressing a carbon dioxide (CO₂) refrigerant; a main heat rejection system for cooling the CO₂ refrigerant discharged from the one or more first compressors; one or more high pressure expansion devices for reducing pressure of the CO₂ refrigerant discharged from the main heat rejection system; a receiver for receiving the CO₂ refrigerant from the one or more high pressure expansion devices; one or more low pressure expansion devices for reducing the pressure of a liquid phase of the CO₂ refrigerant from the receiver; an evaporator for evaporating the liquid phase part of the CO₂ refrigerant from the one or more low pressure expansion devices and thereafter directing the evaporated liquid phase part of the CO₂ refrigerant to a suction side of the one or more first compressors; an auxiliary refrigeration system comprising: an auxiliary compressor arranged to compress at least part of a vapour phase of the CO₂ refrigerant from the receiver to generate compressed CO₂ refrigerant and to direct the compressed CO₂ refrigerant to a heat rejection system, wherein a nominal maximal volume rate of flow of the compressed refrigerant of the auxiliary compressor is less than 10% of the nominal maximal volume rate of flow of compressed refrigerant of the one or more first compressors.
 2. The refrigeration system according to claim 1, wherein the one or more first compressors are arranged to compress the CO₂ refrigerant from the vapour phase of the CO₂ refrigerant to a supercritical phase and wherein the main heat rejection system is arranged to cool supercritical CO₂ refrigerant discharged from the one or more first compressors.
 3. The refrigeration system according to claim 2, wherein the one or more high pressure expansion devices are arranged to reduce the pressure of the CO₂ refrigerant in the supercritical phase, after being discharged from the main heat rejection system, wherein the one or more high pressure expansion devices are further arranged to decompress the CO₂ refrigerant to a subcritical state.
 4. The refrigeration system according to claim 1, wherein the receiver is arranged to receive subcritical CO₂ refrigerant from the one or more high pressure expansion devices.
 5. The refrigeration system according to claim 1, wherein the auxiliary compressor is arranged to either compress at least part of the vapour phase of the CO₂ refrigerant from the receiver to a supercritical phase and thereafter to direct the compressed CO₂ refrigerant to an auxiliary heat rejection system or compress at least part of the vapour phase of the CO₂ refrigerant from the suction side of the one or more compressors to a supercritical phase and thereafter to direct the compressed CO₂ refrigerant to the main heat rejection system.
 6. The refrigeration system according to claim 1, wherein the auxiliary compressor is arranged to direct the compressed CO₂ refrigerant to the main heat rejection system.
 7. The refrigeration system according to claim 1, wherein the main heat rejection system is arranged to allow for cooling of the CO₂ refrigerant discharged from the auxiliary compressor and to allow the CO₂ refrigerant to be led through the main heat rejection system, when the main heat rejection system is off.
 8. The refrigeration system according to claim 1, wherein the auxiliary compressor is arranged to direct the compressed CO₂ refrigerant to an auxiliary heat rejection system.
 9. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system further comprises one or more auxiliary high pressure expansion devices for expanding the CO₂ refrigerant received from the main heat rejection system.
 10. The refrigeration system according to claim 1, further comprising one or more control systems for controlling operation of the auxiliary refrigeration system, including the auxiliary compressor, based on one or more inputs.
 11. The refrigeration system according to claim 1, wherein the refrigeration system comprises one or more measuring devices for measuring a value representative of a pressure and/or temperature of the refrigeration system and providing an input accordingly to one or more control systems of the refrigeration system.
 12. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system comprises a control for controlling a flow of CO₂ refrigerant to the auxiliary compressor.
 13. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system comprises a control comprising one or more pulse width modulation (PWM) valves, and wherein a control system of the refrigeration system is arranged to control the operation of the one or more PWM valves based on one or more inputs representing a pressure and/or temperature of the refrigeration system.
 14. The refrigeration system according to claim 1, wherein the auxiliary refrigeration system is arranged to be powered by an auxiliary power supply separate from a power supply of the main refrigeration system.
 15. A method of operating a refrigeration system in the event a main refrigeration system of the refrigeration system being off, wherein the refrigeration system comprises: a main refrigeration system comprising: one or more first compressors for compressing a carbon dioxide (CO₂) refrigerant; a main heat rejection system for cooling the CO₂ refrigerant discharged from the one or more first compressors; one or more high pressure expansion devices for reducing pressure of the CO₂ refrigerant discharged from the main heat rejection system; a receiver for receiving the CO₂ refrigerant from the one or more high pressure expansion devices; one or more low pressure expansion devices for reducing the pressure of a liquid phase of the CO₂ refrigerant from the receiver; an evaporator for evaporating the liquid phase part of the CO₂ refrigerant from the one or more low pressure expansion devices and thereafter directing the evaporated liquid phase part of the CO₂ refrigerant to a suction side of the one or more first compressors; an auxiliary refrigeration system comprising: an auxiliary compressor arranged to compress at least part of a vapour phase of the CO₂ refrigerant from the receiver to generate compressed CO₂ refrigerant and to direct the compressed CO₂ refrigerant to a heat rejection system, wherein a nominal maximal volume rate of flow of the compressed refrigerant of the auxiliary compressor is less than 10% of the nominal maximal volume rate of flow of compressed refrigerant of the one or more first compressors; wherein the method comprises: detecting a value representing pressure and/or temperature of the refrigeration system by a measuring device and providing an input accordingly to a control system; controlling operation of the auxiliary compressor and if present, a control of the auxiliary refrigeration system, by the control system based on the input.
 16. The refrigeration system according to claim 1, wherein the nominal maximal volume rate of flow of compressed refrigerant of the auxiliary compressor is less than 5% of the nominal maximal volume rate of flow of the compressed refrigerant of the one or more first compressors.
 17. The refrigeration system according to claim 1, wherein the nominal maximal volume rate of flow of compressed refrigerant of the auxiliary compressor is less than 1% of the nominal maximal volume rate of flow of the compressed refrigerant of the one or more first compressors.
 18. The refrigeration system according to claim 1, wherein the nominal maximal volume rate of flow of compressed refrigerant of the auxiliary compressor is less than 0.5% of the nominal maximal volume rate of flow of the compressed refrigerant of the one or more first compressors.
 19. The refrigeration system according to claim 10, wherein the one or more inputs represent a pressure and/or temperature of the refrigeration system.
 20. The refrigeration system according to claim 8, wherein the auxiliary refrigeration system further comprises one or more auxiliary high pressure expansion devices for expanding the CO₂ refrigerant received from the auxiliary heat rejection system. 